Metals are commonly adhered to ceramic materials for various applications. For example, electrochemical sensors, electrolyzers, solar cells and other devices utilize ceramic materials in electrical or electrochemical applications, and often have a metal layer, sheath, or other configuration attached to the ceramic material for collection of electrical current from the ceramic material.
One type of device that utilizes a metal to ceramic connection is a solid oxide fuel cell (SOFC). Fuel cells for combining hydrogen and oxygen to produce electricity are well known. One known class of fuel cells is referred to as solid oxide fuel cells. An SOFC generally consists of a cathode and an anode physically separated by a solid oxide electrolyte, and electrically connected by an external electrical circuit. During operation of an SOFC, oxygen is provided to the cathode of the cell while hydrogen-containing fuel is provided to the anode. Oxygen is catalytically reduced at and diffuses through the cathode to reach the solid electrolyte. The solid electrolyte is permeable to the oxygen anions, which diffuse across the electrolyte to the anode where they combine with hydrogen to form water and release electrons, which flow through the external circuit to the cathode to provide the source of electrons for the catalytic reduction of oxygen, thereby generating electricity.
The cathode of an SOFC must satisfy a combination of criteria, including but not limited to catalytic activity, electrical conductivity (both ionic and electronic conductivity), electronic structure, stability, structural integrity, thermal expansion, and the like, all at operating conditions such as high temperatures in excess of 800° C. A class of materials that have used for SOFC cathode applications is the mixed ionic and electronic conducting (MIEC) materials such as perovskite-type ABO3 oxides. The general chemical formula for perovskite compounds is ABX3, wherein ‘A’ and ‘B’ are two cations of very different sizes, and X is an anion that bonds to both. The native titanium mineral perovskite itself is of the formula CaTiO3. The ‘A’ atoms are larger than the ‘B’ atoms. The ideal cubic-symmetry structure has the B cation in 6-fold coordination, surrounded by an octahedron of anions, and the A cation in 12-fold cuboctahedral coordination. The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations, or both are reduced.
Mixed oxide materials comprising lanthanum (La), strontium (Sr), cobalt (Co), and iron (Fe), also known as “LSCF”, of have been proposed in the prior art as materials for SOFC cathodes due to their high catalytic activity for the oxygen exchange reaction and a high electronic conductivity for current collection. One proposed formulation is characterized by the general formula La1-xSrxCo1-yFeyO3-δ. The physical and chemical properties of this class of materials, such as electrical conductivity, electronic structure, catalytic activity, stability, and thermal expansion coefficient (TEC), have been studied in detail. Generally, electronic and ionic conductivities and catalytic activity are enhanced with increasing values of x and decreasing values of y, whereas there is an opposite tendency for chemical stability.
It has been proposed to use a thin, porous, catalytic, & stable metal film is to establish an equipotential surface and a structure on a perovskite cathode surface that is complementary to the reactions occurring in the air electrode of a SOFC. Proper functioning of metal/oxide interfaces in applications such as SOFC cathodes depends upon electronic interaction (interfacial charge redistribution), and chemical interaction (interfacial atom transport). Further, formation of chemical bonds is a mechanism for charge transfer at the metal/oxide interfaces. The interaction of a metal with a mixed conducting oxide such as LSCF where both electronic and ionic defects are mobile depends on both ionic and electronic conductivities.
To reduce the cathode resistance for intermediate temperature SOFC's noble metals, such as Ag, Pt, Rh, Pd etc. and their alloys have been investigated. Ag and its alloys are attractive due to high permeability, conductivity, and catalytic activity of oxygen through Ag. However, due to the low melting point of Ag at 962° C., Ag—Pd (e.g., with Pd present at 10-30 wt. %) alloy films have been proposed instead. Pd films have been proposed as well. These films can be applied to an LSCF cathode by screen printing a paste of metal powder in an organic resin binder, followed by sintering. However, many of the proposed metal films have been subject to poor adhesion to the ceramic cathode material.
Random large area holes, limited continuity, loose refractory particles, and poor adhesion can decrease the active area for oxygen exchange reaction, thus limiting the contribution of noble metal in reducing cathode resistance. Further in order to draw power from the cell, a metal current collecting screen is typically attached to the electrode using the similar metal powder pastes to those used for the metal film. If the metal has a poor bond with the ceramic, the quality of contact to the current collecting structure is therefore also compromised and can lead to delamination.
It is desirable to obtain metal films and contacts with good adhesion to ceramics, controlled porosity, and a uniform microstructure. Although many of the materials proposed for metal ceramic/metal interfaces have been effective to varying degrees, alternative materials that offer better performance, reliability, cost, or combination of these or other parameters.